1. School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK;
2. Institute of Biophysics Academy of Sciences of the Czech Republic, CZ–61265 Brno, Czech Republic;
3. Jodrell Laboratory, Royal Botanic Gardens Kew, Richmond, Surrey TW9 3DS, UK;
4. Laboratoire de Biologie Cellulaire, INRA – Centre de Versailles, F–78026 Versailles, France